HCl and Br2-MeOH etching of Cu2ZnSnSe4 polycrystalline absorbers

Cu2ZnSnSe4 solar cells made from absorbers etched in HCl and Br in methanol (Br2-MeOH) are studied. The absorbers show surface compositions, different from bulk compositions, as revealed by secondary ion mass spectrometry (SIMS) depth profiles. They indicate the presence of secondary phases where the most prominent are a Zn related phase and a Cu, Sn related phase. The secondary phases can be removed by etching in HCl or Br2-MeOH. The absorbers are analyzed before and after etching by scanning electron microscopy and SIMS, and solar cells by current–voltage measurements. These results indicate that the Cu, Sn related phase is strongly detrimental to solar cell devices by reducing drastically open-circuit voltage (VOC) and fill factor (FF) and is etched by Br2-MeOH, but not by HCl. In fact solar cell results improved from about 4% to above 5%. All solar cell parameters improved slightly for HCl etching but for Br2-MeOH etching we observe a significant increase of VOC and FF. An efficiency of 5% was obtained in both cases and 5.8% efficiency is the best device obtained after Br2-MeOH etching.

► Secondary phases could be removed by HCl and Br2-MeOH etching.
► 5% solar cell efficiencies for Cu2ZnSnSe4 have been achieved with both etchants.
► The open circuit voltage increases for Br2-MeOH etched absorbers.
► A Cu, Sn related secondary phase at the interface is detrimental to solar cells.
► 5.8% efficiency has been achieved with Br2-MeOH etching.